The work of rivers and streams.

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Rivers and streams are always at work on the land, destroying rock and soil (erosion), washing them away (transportation), and putting them down some place else (deposition). The first two processes — erosion and transportation — wear down the land, changing highlands into lowlands. The last process — deposition — builds up the land. Together these three processes keep a balance between the high places and the low places of the earth. As a standing liquid, water changes rocks and soil chemically through its dissolving action. As ice, it breaks up rocks mechanically. But when it moves as ice or flows as water, it does its work of erosion. Streams and rivers, like mountains, can be described as young or old. Streams flowing down steep slopes are considered young. They have fairly straight courses. Young streams are still cutting into their streambeds and forming their valleys, which are narrow and V-shaped. The land between stream valleys is usually high and broad. As rivers become older, their currents slow down. Less erosion takes place, and more materials are deposited along their banks. Streambeds gradually widen and flatten out. Then floodplains build up. The soil of floodplains is extremely fertile. The world's four earliest civilizations developed along the fertile floodplains of the Tigris-Euphrates, Nile and Indus rivers. As the rivers advance into old age, they begin to change course. Instead of flowing straight through the land, they move from side to side, forming wide bends. At times of flash floods, old bends may be cut off. As new bends form, the old bends may remain as lakes.